How to change or delete a defined cell name

I have a spreadsheet that I define a name to a give cell. For example, Cell A10 is defined as Gross_Margin. Once defined, how do I:

1) Change that defined name? For example, if I want to change it to Gross_Margin_Percent, how do I do that?
2) Delete that defined name? In this case, I want to delete it all together?


The only thing that it appears that I can do it to add a new cell name. But the problem is that when I try to access it, then all names are listed!

Thanks in advance for your help!
Jim